As is well known, a shadow mask CRT is a color reproducing cathode-ray tube of the kind comprising, within an evacuated envelope, an image screen provided with a plurality of groups of elemental phosphor areas, the groups of phosphor areas being emissive of red, green and blue light respectively and being interspersed so as to form repetitive clusters of areas including one area from each group, electron gun means for projecting a corresponding plurality of electron beams toward the image screen, deflection means for causing the electron beams to scan the image screen in synchronism, and a mask (the shadow mask) disposed adjacent the screen between the latter and the electron gun means and having a plurality of apertures so arranged as to constrain each beam to strike the elemental phosphor areas of only one respective group.
Shadow mask CRTs have long been used in the field of domestic color television, and their construction and operation is very well known to those skilled in the art. One example of a typical shadow mask CRT is described in U.S. Pat. No. 3,146,368.
Although U.S. Pat. No. 3,146,368 describes a construction of shadow mask CRT in which the elemental phosphor areas are in the form of circular dots clustered in triads of red, green and blue light-emitting phosphors, these areas may take other shapes with a corresponding shape of the apertures in the shadow mask. Thus, the elemental phosphor areas may be in the form of clusters of rectangles, hexagons or other geometric shapes.
Furthermore, a recent and now well-established form of shadow mask tube uses narrow vertical phosphor stripes each of which extends the full height of the image screen. In this case, each cluster of elemental phosphor areas constitutes a set of red, green and blue vertical phosphor stripes and the corresponding shadow mask (alternatively referred to as an aperture grill in this type of tube) comprises a large number of vertical slits also extending the full height of the screen. A shadow mask CRT of the latter type is referred to in U.S. Pat. No. 3,666,462, particularly with reference to FIG. 5. In either case the image screen may comprise the inside surface of the CRT faceplate itself, or a separate transparent support behind the faceplate.
In the aforementioned U.S. Pat. No. 3,146,368, each of the elemental phosphor areas is spaced on the image screen from all adjacent such areas and the apertures in the shadow mask are individually larger than the elemental phosphor areas so that each beam striking any given elemental phosphor area additionally falls on a portion of the screen which spaces that area from adjacent areas. In particular, a negative tolerance guard band arrangement is described in which circular phosphor dots are used and the electron beam not only falls upon the dot in any given case, but also upon an annular portion of the screen immediately surrounding the dot, a black light-absorbing material known as a black matrix being provided over substantially the entire area of the screen not occupied by the phosphor dots.
The advantage of this arrangement is that the black matrix intermediate the dots absorbs ambient light and increases the contrast of the image. The negative tolerance guard band black matrix technique has also been applied to the aperture grill type of shadow mask CRT, see for example, U.S. Pat. No. 4,267,204, with the vertical slits in the grill being wider than the phosphor stripes and the latter being separated from the adjacent stripes by intermediate stripes of light-absorbing material. In this case the electron beam passing through any given aperture falls substantially centrally on the relevant phosphor stripe with the opposite lateral edges of the beam falling on the light-absorbing material on either side. In modern shadow mask CRTs the light-absorbing material or black matrix comprises graphite of sub-micron particle size.
The long-established development of shadow mask tubes such as those described in U.S. Pat. Nos. 3,146,368 and 3,666,462 for domestic television, with their consequent high reliability and relatively low cost, has led to their use as video display units in multi-color computer graphics applications. Essentially, the shadow mask tubes used in computer graphics are the same as those used in domestic television, except that for high resolution graphics both the number of individual elemental phosphor areas on the image screen and the precision of the deflection circuitry is increased as compared to the domestic tube. Nevertheless, whether the tube is for high resolution graphics or low resolution graphics (in which case a domestic-grade tube can be used), the fundamental principles of construction and operation are well known.
A common requirement in interactive computer graphics is the ability to provide user feedback by the use of a so-called light pen which contains a photosensitive device responsive to light emitted by the CRT display for providing a feedback signal to the display control unit. It is important in such applications that the light pen reliably "triggers" in response to any light emissive portion of the displayed image at which the pen is pointed at any given time.
The light pen may employ a PIN diode for high sensitivity, and in order to trigger such a light pen reliably it is necessary that the phosphors employed on the screen have a fast transient (rise time). This is a particular problem for the red phosphor, since when the color graphics display is capable of displaying over one million picture elements on a 20" diagonal screen, even the widely used industry standard rare earth type P22R red phosphor is not fast enough to activate the highly sensitive PIN diode.